<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script type="text/javascript">
    var a=prompt("请输入年龄")
    if(a>=18){
        alert("欢迎光临，聚缘网吧给你家的温暖！")
    }else {
        alert("长大再来,网吧不欢迎你")
    }
</script>
</body>
</html>